It's hard to exactly pinpoint the largest battle fought by the Roman Empire because of historical inaccuracy and how we can't actually estimate the size of a military force exactly. Therefore, many historians disagree on what was the "largest battle fought" in the history of Rome.
Since your question is very broad, I'll talk about two battles that many consider the largest fought.
The Battle of Philippi in 42 BC had probably the most Roman troops present at a single battle. Following the assassination of Julius Caesar, two of the main conspirators; Brutus and Cassius fled Rome to take control of the eastern provinces. They raised a large army consisting of the eastern legions and levies recruited from local kingdoms allied to Rome. In estimate, this army probably consisted of around 17 legions: with approximately 17'000 cavalry and 100'000 men.
To counter Brutus and Cassius, the members of the Second Triumvirate (a legal committee of three men, established to avenge the death of Julius Caesar.) in Rome, Mark Antony, Octavian, and Marcus Lepidus, raised their own army to defeat the conspirators. This force approximately totalled 19 legions: with 33'000 cavalry and over 100'000 men.
So overall, the Second Triumvirate raised a force of 223'000 troops if auxiliary numbers matched legionary numbers. Whilst the conspirators raised a force of 187'000 with auxiliary numbers added.
That's a total amount of 410'000 men fighting in one battle. A pretty impressive number.
However, the Battle of Philippi can not be considered the biggest battle, only the battle with the largest amount of Romans present. The title of largest battle likely goes to Alesia, even though the number of Gauls at the battle is up to debate.
A little bit of background information. The Battle of Alesia was a major confrontation in 52 BC during the Gallic Wars. In 58 BC, Julius Caesar raised an army and began a series of campaigns to pacify Gaul and finally bring it under control. Over the next couple of years, he managed to defeat several tribes and bring large amounts of land under conquest, however a revolt in 54-53 BC destroyed the Fourteenth Legion. Deprived around a quarter of his troops and with reinforcements from Rome unlikely due to political upheaval and intrigues, it looked like the Gaelic Wars would be lost for the time being. After months of relentless campaigns, however, Caesar and his force finally managed to re conquer Gaul.
However, the win caused revolt among the tribes of Gaul. Leaders met and decided that Vercingetorix would lead the united Gaelic army against the Romans. Waves of violence caused large numbers of settlers, soldiers and merchants to die by the hands of Gaelic men. After hearing of this, Caesar mobilised his army and moved across the Alps to strike at the Gauls at Alesia.
Julius Caesar faced Alesia with a total of 60'000 men, but this military force was totally dwarfed compared to Vercingetorix's. Their were thought to be 80'000 Gauls in Alesia, with an estimated range of 100'000-250'000 men in the relief army. Although according to the Greek historian Plutarch, 300'000 men.
The total number of men here would add up to between 240'000-440'000.
Hope this helped!
My sources are: http://militaryhistory.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=militaryhistory&cdn=education&tm=885&f=00&su=p284.13.342.ip_&tt=11&bt=5&bts=0&zu=http%3A//www.unrv.com/fall-republic/siege-of-alesia.php http://militaryhistory.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=militaryhistory&cdn=education&tm=905&f=00&su=p284.13.342.ip_&tt=11&bt=5&bts=0&zu=http%3A//www.historyofwar.org/articles/battles_alesia.html http://militaryhistory.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=militaryhistory&cdn=education&tm=4&f=00&su=p284.13.342.ip_&tt=11&bt=5&bts=0&zu=http%3A//www.readeasily.com/jacob-abbott/00158/001580068.php http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Plutarch/Lives/Brutus*.html http://militaryhistory.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=militaryhistory&cdn=education&tm=61&f=00&su=p284.13.342.ip_&tt=11&bt=5&bts=0&zu=http%3A//www.unrv.com/fall-republic/philippi.php